In terms of classical physics, "mass" is a measure of inertia, i.e., it is a measure of the opposition that a body offers to any attempts at changing its state of motion. The greater the mass of a body, the lower its acceleration under the action of an applied force. On Earth, the mass of a body can be determined by measuring its weight, where weight is the gravitational force acting on the body. If the acceleration due to Earth's gravity, a, and the weight, F, are known, the mass, m, can be determined from Newton's second law of motion, which in equation form is F=ma.
A problem with conventional methods of obtaining mass by measuring weight for some conditions is that they require gravity, that is, a condition of not being "weightless". In environments where the acceleration of gravity is apparently zero, such as in a spacecraft orbiting Earth, where it is zero, such as deep in intergalactic space, or where it is not the standard value, such as on the moon, some means for measuring mass other than the typical, conventional methods of measuring weight on Earth must be used. In classical physics, mass is defined such that a given body has a value that remains constant under all of the conditions described above.
Equipment is in use today for measuring mass under microgravity conditions, such as during space travel. One type of such equipment is a simple spring-mass system, wherein the object whose mass is to be measured is oscillated. The relationship between the mass of the object and the period of oscillation is used to obtain the mass value, using the principle that for a spring-mass oscillating system, the square of the period is related to the mass and the spring constant of the spring. A second type of such equipment imparts angular motion to a fluid by a rotary impeller in a housing. The rotation of the fluid creates centrifugal forces and fluid pressures. The centrifugal pressure is measured with a pressure transducer and converted to a mass value.
A third type of instrument to measure mass is through utilizing principles of angular simple harmonic motion. The object whose mass is to be measured is placed on a platform having torque restoring means. The platform is rotated from an equilibrium position and released, such that it undergoes angular simple harmonic motion. The period of oscillation is measured and used to calculate the mass of the object on the platform.
A problem with existing systems for measuring mass under microgravity conditions is that they require calibration using a large number of known masses over the measurement range. For example, reasonable approximations can be obtained with the spring-mass system if it is assumed to be a single degree of freedom oscillator and an ideal linear relationship exists between the square of the period of oscillation and the mass of the object. However, over even small mass ranges of the equipment, that relationship varies from the ideal sufficiently that errors prevent achieving consistently high accuracy. So, the variations from the ideal can be reduced by an increase of the number of calibration masses.
Similar problems exist for the rotary impeller system in that there are variations about the expected linear relationship between mass and pressure. Therefore, the rotary impeller system requires calibration for a set of masses over closely spaced intervals. In addition, calibration over the full mass range is required for accurate results. Recalibration is also required under some conditions. This consumes time, promotes errors, and requires availability of calibration masses. Budgeting of time and weight are important considerations for spaceflight missions.
The rotary impeller system also has the disadvantage of not being adaptable to measuring items other than liquids. It particularly is not well suited for measuring the masses of solid items, especially if they are large.
Also, the physical sizes of these types of systems for use in microgravity are large. Obviously, that is a negative factor for many situations where mass must be measured. One spring-mass system designed to operate in microgravity also requires significant additional equipment for it to be able to operate in a gravity environment as on Earth. Operation on Earth is very desirable for checkout, testing, and verification purposes.
The rotary impeller system is complex to operate and requires lengthy time interactions by the operator. The cycle time to perform a mass measurement is long. The equipment has difficulty removing all of a sample after a measurement, which will cause an error for the following sample measurement. It also has reduced accuracy for smaller samples. It has poor maintainability, repairability, and reliability.
The angular simple harmonic motion system has not received significant development and has not been tested or operated under microgravity conditions. This increases the risks of improper and inadequate operation.